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Geology is the Way

Lamprophyres

Lamprophyres (from the Greek lampros, ‘glistening’ + porphyros ‘porphyry’) are rare and poorly understood ultrapotassic hypabissal rocks that occur only as dikes. Even though they share similarities with some kimberlites and lamproites, they cannot be separated easily from other plutonic rocks using the QAPF diagram or the TAS classification. These rocks consist largely of porphyric femic minerals (color index M = 35-90%) and contain feldspars and/or feldspathoids that are confined to the groundmass. They usually contain biotite, phlogopite, and/or amphibole, and sometimes clinopyroxene and olivine. Igneous minerals can be associated with hydrothermal minerals, like calcite and zeolites, and can show widespread evidence of hydrothermal alteration. Through the years, several names have been proposed to classify different lamprophyre types, producing a “legion of obscure rock types named after equally obscure European villages” (Rock, 1990). At present, the IUGS – Subcommission of Systematics of Igneous Rocks does not endorse most of these local terms, encouraging the classification that follows. An in-depth description of lamprophyres and their local names is available at Alexstrekeisen.it.

Classification
The classification of lamprophyres is based on:
1) the most abundant light-colored constituents (alkali feldspar, plagioclase, or feldspathoids);
2) the predominant mafic minerals (biotite, hornblende, augite, olivine, or brown amphibole).

When alkali feldspar > plagioclase and no foids are present, a lamprophyre is:
-a minette if biotite > hornblende, augite and/or olivine;
-a vogesite if the mafic minerals are hornblende, augite, and/or olivine.

When plagioclase > alkali feldspar and no foids are present,:
-a kersantite if biotite > hornblende, augite and/or olivine;
-a spessartite if 
the mafic minerals are hornblende, augite, and/or olivine.

If feldspathoids are present, a lamprophyre is:
-a sannaite, if alkali feldspar > plagioclase and feldspars > foids;
-a camptonite if plagioclase > alkali feldspar and feldspars > foids;
-a monchiquite if there are no feldspars, but only glass or foids.

Sannaite, camptonite, and monchiquite typically contain brown amphibole, Ti-augite, olivine, and/or biotite.

Mesozoic lamprophyre dyke crosscutting Ordovician sediments near Leading Tickles, Northern Newfoundland, Canada. Photo by Alexander Peace.

Oligocene lamprophyre dyke (minette) located beside Red Rock Highway, Navajo Volcanic Field, New Mexico, USA. Photo by James St. John.

Lamprophyre dyke crosscutting granodioritic rocks. The bright crystals are biotite. Depending on the most abundant feldspar in the groundmass, this could be a minette or a kersantite. Neves, Sudtirol, Italy. Photo: Samuele Papeschi/Geology is the Way.

Lamprophyre with amphibole (black) and altered olivine (brown) from Scania, Sweden. Photo by Hildegard Wilske.

Precambrian Monchiquite lamprophyre. Michipicoten River Bridge, Wawa, Michipicoten Greenstone Belt, Ontario, southeastern Canada. Photo by James St. John

Minette from the Precambrian of Canada, with dark crystals of biotite and white patches of orthoclase feldspar. This rock contains micro-diamonds. Paleoproterozoic Akluilâk Dike System,Gibson Lake area, District of Keewatin, Northwest Territories, northern Canada. Photo by James St. John.

Camptonite lamprophyre from Campton Falls, Grafton County, New Hampshire, USA on display at the Geology Department, Wittenberg University, Springfield, Ohio, USA. Photo by James St. John.

References
Mitchell, R. H. (1994). The lamprophyre facies. Mineralogy and Petrology51(2), 137-146.
Rock, N. M. (1987). The nature and origin of lamprophyres: an overview. Geological Society, London, Special Publications30(1), 191-226.
Rock, N. M. (2013). Lamprophyres. Springer Science & Business Media.
Tappe, S., Foley, S. F., Jenner, G. A., & Kjarsgaard, B. A. (2005). Integrating ultramafic lamprophyres into the IUGS classification of igneous rocks: rationale and implications. Journal of Petrology46(9), 1893-1900.
Woolley, A. R., Bergman, S. C., Edgar, A. D., Le Bas, M. J., Mitchell, R. H., Rock, N. M., & Scott Smith, B. H. (1996). Classification of lamprophyres, lamproites, kimberlites, and the kalsilitic, melilitic, and leucitic rocks. The Canadian Mineralogist34(2), 175-186.

        

See also
Lamprophyres – Alexstrekeisen.it

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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